This method relates, in general, to CDMA (Code Division Multiple Access) spread spectrum receivers, and more specifically, to fast acquisition GPS (Global Positioning System) receivers.
Spread spectrum communication in its basic form is a method of taking a data-signal that is used to modulate a sinusoidal carrier and then spreading its bandwidth to a much larger value, e.g. in a global positioning system (GPS) application by multiplying a single-frequency carrier by a high-rate binary (xe2x88x921,1) pseudo-random noise (PRN) code sequence that is known to GPS users. Thus, the signal that is transmitted includes a data component, a PRN component, and a (sinusoidal) carrier component.
At the receiver, a synchronized replica of the transmitted PRN code is required to de-spread the data sequence. Initial synchronization, called acquisition, is followed by fine synchronization, which is called tracking.
The present invention relates to acquisition of a spread spectrum signal. Acquisition is the process by which the replica PRN code is synchronized (to within a small timing offset) with the code conveyed by the received signal either for the first time or after losing a previously acquired signal, and also by which the carrier frequency of the received signal is determined. Thus, to acquire a signal, an acquisition system must accurately determine any frequency-shifting of the received signal from the transmitted frequency in order to accurately wipeoff (remove) the carrier signal. Frequency-shifting can be caused by relative motion of the transmitter and receiver (Doppler-shifting) as well as by clock inaccuracies (so that a transmitter and receiver sometimes do not agree on what is in fact the same frequency). The carrier frequency-shifting results in a modulation of a code component after carrier wipe-off in the receiver. Thus, in acquiring a signal, it is also necessary that the replica code sequence be not only time-aligned with the received code sequence, but also modulated to compensate for the frequency-shifting so as to fully eliminate the PRN sequence and leave behind only the data conveyed by the received signal. The acquisition process is therefore a two-dimensional search, a search both in code phase and in frequency.
For GPS signals the search interval in the frequency domain can be as large as +/xe2x88x926 kHz. In addition, the phase of the received code relative to the replica can be any possible value of code phase, due to uncertainties in position of the satellite and time of transmission of the received signal. A PRN code period is typically 1023 chips, the term chips being used to designate bits of code conveyed by the transmitted signal, as opposed to bits of data. Thus, the acquisition module of a receiver must search a 12 kHz-wide interval with 1023xc3x97ks different code phases, where ks denotes the number of samples per chip.
Ordinary GPS receivers, i.e. those designed only for operation with unobstructed satellites, search for the frequency shift with a granularity of around 1 kHz. Thus, such receivers must search 12xc3x971023xc3x97ks different code/frequency combinations.
A GPS receiver designed for indoor operation must have an operating mode with equivalent noise bandwidth on the order of 10 Hz in the acquisition stage. Even with an equivalent noise bandwidth as small as 10 Hz though, for reliable tracking some post-detection filtering must still be performed as well as some further refining of the value determined for the carrier frequency in the acquisition stage. The granularity of 10 Hz requires that the receiver search 1200xc3x97ksxc3x971023 different code/frequency combinations and makes the sequential search so time-consuming as to be unrealistic, motivating the use of parallel and fast search methods.
Some existing methods often intentionally modulate the signal being acquired, sequentially or in parallel, to advance the search in frequency.
Existing methods are promising and useful in many situations, but what is needed is a spread spectrum acquisition system that performs the code/frequency acquisition search in a way that is computationally efficient and still as accurate as is needed for a given application. In other words, what is needed is a way to tailor an acquisition system so as to be able to balance the competing requirements for accuracy and speed appropriately for a given application.
Accordingly, the present invention provides a method, a corresponding apparatus, and a corresponding system, for acquiring a received spread spectrum signal, the received signal having a carrier component at a carrier frequency, a code component having a code period, and a data component, the acquiring including matching the phase of a replica of the code component to the phase of the received code component and also determining a possible shift in the carrier frequency away from a nominal carrier frequency, the acquiring having an intrinsic bandwidth that is substantially the inverse of a predetermined fraction of a code period and also having a design bandwidth that is a fraction of the intrinsic bandwidth including possibly the entire intrinsic bandwidth, the method comprising the steps of: obtaining samples of the received signal spanning one code period after the received signal is demodulated at the nominal carrier frequency, the obtaining of the samples being performed at a sampling rate, the samples being distributed over a number of sections as a predetermined number of samples per section, the inverse of the product of the number of sections and the code period determining the intrinsic bandwidth; determining a first frequency from among an ordered set of first frequencies spanning a frequency interval to be searched for the possible shift in the carrier frequency; mixing a sinusoid at the first frequency with the samples of the received signal, to provide a once-mixed received signal segment; assigning a value to the replica code phase, the assigning of a replica code phase amounting to associating a value of the replica code phase with a given position of the replica compared to the received signal; performing a series of section correlations with the sections of replica code of the once-mixed received signal segment, the samples spanning one code period, each section correlation being calculated using one section of the replica, phase-shifted by the replica code phase, the first correlation in the series being performed over the samples spanning the first section, and so on; determining an ordered set of second frequencies, the second frequencies lying in the interval between the selected first frequency and the next first frequency in the ordered set of first frequencies; coherently combining each of the section correlations using for the first section correlation a compensating factor including a sinusoid derived from the first second frequency in the ordered set of second frequencies, and so on, to provide a correlation with a code period of the replica at the code phase compensated by a compensating factor including a sinusoid derived from each of the frequencies in the ordered set of second frequencies; and repeating, for each further replica code phase in the ordered set of replica code phases, the series of section correlations and the coherent combining of the section correlations for each replica code phase a number of times equal to the number of frequencies in the ordered set of second frequencies using a different compensating factor for each coherent combining, the compensating factor for the first coherent combining including a sinusoid derived from the first second frequency in the ordered set of second frequencies, and so on, to determine a correlation at each of the further replica code phases and, for each of such replica code phases, at each of the second frequencies; wherein the parameter of design bandwidth is adjusted so as to alter characteristics of the acquisition.
In a further aspect of the invention, to achieve a desired tradeoff between design bandwidth in the sense of a portion of the frequency separation between successive first frequencies, cancellation of aliasing introduced by integration of the received signal with the replica code in computing a correlation of the received signal with the replica code, and frequency resolution, two parameters from among the set of three parameters consisting of the design bandwidth B, the number of second frequencies Nf, and frequency resolution fres, indicating the spacing between adjacent frequencies in the ordered set of second frequencies are independently selected, and the other parameter is determined based on the relation,
xe2x80x83B=fresxc2x7Nf
In yet another further aspect of the invention, the correlations used in the acquisition process are continued over several code periods, the correlations over the several code periods being coherently combined (i.e. taking into account both the sign and the magnitude of each term being combined) using compensation factors based on different trial second frequencies.